Generally, a timing gear represents a gear or pulley which is attached to a front end of a crank shaft and a cam shaft installed in an engine part of a vehicle, so that a valve opening and closing timing is synchronized with a rotation of a crank shaft. There is provided a chain type in which a chain is used to connect a gear of a crank shaft and a gear of a cam shaft. In a belt type, a timing belt is used. In a gear type, a spur gear is used with its accurate transmission of a driving force. In the case that a distance between the shafts is large, the chain type and the belt type have been generally used. The use of a low cost belt type increases as compared to expensive chain type.
A V-shaped rib engaging with the gear is formed at a surface of the timing belt. The belt is made of a certain fiber, which is not flexible, such as a rubber or a glass fiber. With its light and low noise characteristic and no need of lubricant, the belt has been used for a reciprocation type engine.
FIG. 1 is a view illustrating a conventional coupling structure of a timing belt, FIG. 2 is a cross sectional view and stress analysis result of a conventional trapezoid thread shaped timing belt, FIG. 3 is a cross sectional view and stress analysis result of a conventional semicircular thread shaped timing belt, and FIG. 4 is an enlarged view illustrating a surface of a jacket of a conventional timing belt.
As shown in FIG. 1, a timing belt 40 is connected over a cam shaft sprocket 10, a crank shaft sprocket 20, and an oil pump sprocket 30 for thereby connecting the corresponding sprockets. A tensioner pulley 11 contacts with the timing belt 40 connected between the camshaft sprocket 10 and the crankshaft sprocket 20. An idler pulley 31 contacts the timing belt 40 connected between the oil pump sprocket 30 and the camshaft sprocket 10.
With the above-described construction, when the engine starts, the valve opening and closing timing is synchronized with the rotation of the crankshaft.
In FIG. 1, reference numeral 12 represents a timing mark, 13 represents a tensioner arm, and 14 represents an automatic tensioner.
As shown in FIGS. 2 and 3, in the timing belt 40 disposed over the sprockets 10, 20 and 30 and the pulleys 11 and 31, a plurality of threads 43 are formed at the belt body 42 at regular intervals in the direction (namely, horizontal direction) of the crank shaft or the cam shaft. A plurality of gear surfaces are formed on the outer surfaces of the cam shaft sprocket 1 and the crank shaft sprocket 20 to correspond with the threads 43.
As shown in FIGS. 2 and 3, the threads 43 of the conventional timing belt are formed in a trapezoid or semicircular shape. A jacket formed of nylon is covered on the threads 43 for thereby fabricating the conventional timing belt.
However, in the case of the timing belt 40 having the above-described threads 43, the stress of the sprockets 10, 20 and 30 is focused at the corner portion 45 contacting with the threads 43 and the belt body 42, so that as shown in FIG. 4, the jacket (formed of nylon having high durability) may be worn out based on the repeated contacts with the sprockets 10, 20 and 30. As shown in FIGS. 2 and 3, the stress is supported only by the rubber material provided at the lower side of the jacket, so that cracks may occur at the corner portion 45 by fatigue stress. In addition, the threads may be disengaged for thereby decreasing the life span.
In addition, in the case of the timing belt, as the timing belt rotates, when the belt threads 43 are engaged at the threaded portions of the sprockets 10, 20 and 30, the air cannot be efficiently discharged from the threaded portions between the belt threads 43 and the sprockets 10, 20 and 30, so that noises and vibrations increase.